It means ..the Corporal who was asking ques. to him..did'nt expected from him, as the man was very intelligent  and also know about rifles ...
The line on the Corporal's face is also meant that he was shocked by the answers given by the Man.
When Quelch added to his lectures and corrected him,in front of all his students,it made the corporal feel insulted.the whole class went silent.corporal was burning with the fire of anger and revenge towards the 'prof.'and for the same reason his brown face turned red-a sign of anger
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